Page 279 - Introduction to Marine Engineering

Chapter 74_

Electrical equipment

The complete electrical plant on board ship is made up of power
generation equipment, a distribution system and the many power
utilising devices. Electricity is used for the motor drive of many
auxiliaries and also for deck machinery, lighting, ventilation and air
conditioning equipment. A constant supply of electricity is essential for
safe ship and machinery operation, and therefore standby or additional
capacity is necessary together with emergency supply equipment.
Emergency equipment may take the form of an automatically starting
emergency alternator or storage batteries may be used.
The complete range of electrical equipment will include generators,
switch gear for control and distribution, motors and their associated
starting equipment and emergency supply arrangements.

Alternating or direct current
Alternating current has now all but replaced direct current as the standard
supply for all marine installations. The use of alternating current has a
number of important advantages: for example, reduced first cost, less
weight, less space required and a reduction in maintenance require-
ments. Direct current does, however, offer advantages in motor control
using, for example, the Ward-Leonard system which provides a wide
range of speed.

Machine rating

Motors and generators, both d.c. and a.c., are rated as Continuous
Maximum Rated (CMR) machines. This means they can accept a
considerable momentary overload and perhaps even a moderate
overload for a longer duration.
Temperature affects the performance of all electrical equipment and
also the useful life of the insulation and thus the equipment itself. The
total temperature of an operating machine is a result of the ambient air